Field of the Invention
The present invention relates to an image forming apparatus configured to form an image on a sheet.
Description of the Related Art
An image forming apparatus employing electrophotography develops a latent image formed on a photosensitive drum into a visible image. This visible image (toner image) is transferred onto a sheet at a transfer unit by using an electrostatic force. Then, the image transferred onto the sheet is fixed to the sheet by heat at a fixing unit, whereby an image is formed on the sheet.
In the image forming apparatus, there is provided a sheet detection unit for detecting the sheet conveyance timing inside the image forming apparatus and the conveyance attitude. For example, to stabilize the sheet conveyance between the transfer unit and the fixing unit, there is arranged, between the transfer unit and the fixing unit, a loop detection unit for detecting the loop amount of a sheet (See Japanese Patent Application Laid-Open No. 2006-072253). The loop detection unit is composed of a loop detection lever configured to rotate when pushed by a sheet, and a loop detection sensor (photo interrupter) configured to generate a signal corresponding to the position of the loop detection lever. Based on a signal from the loop detection sensor, speed control is performed at the fixing unit so that the attitude of the sheet may be stabilized. A loop sensor lever directly contacting the sheet is arranged so as to be held in contact with the non-image surface of the sheet. This is due to the fact that, if the loop sensor comes into contact with the image surface side, the unfixed toner image will be disturbed by the loop sensor lever.
Another example of the sheet detection unit, a detection lever of a post-fixing detection sensor provided on the downstream side of the fixing unit is also arranged so as to be in contact with the non-image surface of the sheet. This arrangement is made for the purpose of preventing the toner image from being disturbed by the detection lever.
In this context, there exists an apparatus equipped with a door that can be opened and closed with respect to an apparatus main body provided with a detection sensor, and a sensor lever is rotatably mounted on the door (Japanese Patent Application Laid-Open No. 2008-70522 and Japanese Patent Application Laid-Open No. 2-248964). With this configuration, the detection accuracy depends on variation in the position of the door when it is closed (the relative position thereof with respect to the apparatus main body).